One major obstacle in using allogeneic islet transplantation for treatment of type I diabetes mellitus is the immune response against donor-derived alloantigens and islet autoantigens. We propose to develop novel strategies to prevent islet transplantation rejection by the manipulation of two recently described costimulatory pathways, LIGHT-HVEM and B7-H1. The central hypothesis of this proposal is that selective manipulation of LIGHT-HVEM and B7-H1 costimulatory pathways induces tolerance of T cell responses specific to transplanted islets without systemic immune suppression. Our preliminary data demonstrated that blockade of LIGHT-HVEM costimulation by systemic administration of soluble LTaR-Ig fusion protein inhibits progression of acute graft-versus-host diseases in mouse models and generates long-lasting alloantigen-specific T cell anergy. Additionally, we have found that several types of human cancers express B7-H1 and use this mechanism to delete activated T cells in order to escape immune attack. In this proposal, we will use lentiviral vector as a delivery vehicle to modify islet cells to secrete LTaR-Ig. Optimal transduction protocols will be established and the survival and function of genetically modified islet cells in mouse models will be evaluated. In addition, we will test the effect of lentivirus-mediated expression of B7-H1 to generate an immune inhibitory microenvironment around the transplanted islets. We will also explore strategies to combine these two approaches to achieve maximal therapeutic effects. Finally, we will address the mechanisms underlying graft acceptance vs. rejection using a traceable monoclonal T cell system. It is anticipated that these studies will provide a foundation to develop new approaches for islet transplantation and facilitate the treatment of type 1 diabetes mellitus.